The liquid crystal display has become ubiquitous and well known, driven in part by popular applications such as laptop personal computers, car navigational displays, and flat panel displays for personal computers. In each of these applications, a column driver circuit enables the operation of each liquid crystal display unit. Liquid crystal displays comprise a plurality of individual picture elements, called pixels, which are uniquely addressable in a row and column arrangement. The column driver circuitry provides driving voltages to the columns of the liquid crystal display. In a typical application, a 13.3-inch extended graphics array (“XGA”) liquid crystal display comprises 1024 3-color columns, for a total of 3072 individual columns. In a representative arrangement, these columns are driven by eight 384-column driver chips.
The physics underlying liquid crystal display technology calls for an alternating polarity in the driving voltage. For example, if a column of the display is driven at +5 volts for a specific period of time, then this same column is driven at −5 volts during the subsequent time interval. In such an arrangement, the peak to peak voltage is 10 volt, but the sum of the individual driving voltages for any given cycle is 0 volt.
Liquid crystal displays (“LCDs”) are manufactured in a variety of sizes and display formats. The thin film transistor (“TFT”) technology LCDs, in which each picture element, or pixel, is driven by one to four transistors, must be driven with voltages that sum to zero over successive cycles. Failure to so drive the display causes the display device to degrade until it becomes unusable. A variety of methods can be used to drive the LCD at alternating polarities. Polarity inversion comprises switching the polarity of the voltage applied to drive the columns of the LCD over time to obtain an average of approximately 0 volt over time. Exemplary methods for alternating polarities include frame inversion, line inversion, column inversion, and dot inversion.
In frame inversion, each pixel element in the entire panel is driven with a similar polarity in a given frame. In a subsequent frame, each pixel element is driven with a polarity opposite to that used to drive the previous frame. One characteristic of frame inversion is that the polarity reversal must occur at a sufficient rate in order to reduce “flicker,” the appearance of a changing image. This arises due to slight variations in the color and/or intensity of the pixels in the display depending upon the polarity the pixels are driven. Changing the polarity of all pixels in the display at the same time causes the slight variation to occur at once all over the display screen, which can be noticeable to the eye if the rate of change is not quick enough. Frame inversion can have the benefit of lower power consumption and less complex display driver circuitry due to the uniformly timed polarity changes.
The method of line inversion drives all the pixels of every other row (line) in the display at opposite polarities at a given period of time. In a subsequent period of time, every pixel element in every other row is driven with a polarity opposite to that used to drive the row in the previous time period. Line inversion methods provide both temporal and spatial averaging of polarity related pixel variations, giving a more uniform appearance to the display image. Power consumption is greater in embodiments using line inversion than in embodiments using frame inversion. In line inversion methods, the column driver switches the polarity of the voltage applied to the column line at the time that the information displayed on each line is updated.
In the column inversion method, all the pixels of every other column in the display are driven at opposite polarities at a given period of time. In a subsequent period of time, each pixel element in every other column is driven with a polarity opposite to that used to drive the column in the previous time period. In column inversion methods, the column driver switches the polarity of the voltage applied to the column at each successive cycle, and maintains every other column at an opposite polarity. Column inversion methods are characterized by a relatively low power consumption than that provided by line inversion methods. Further, column inversion methods can provide superior spatial averaging characteristics due to the larger number of columns than rows in many display geometries.
In the dot inversion method, each individual pixel is driven at an opposite polarity from its neighbor, both along the row and along the column directions at a given period of time. In a subsequent period of time, each pixel is driven with a polarity opposite to that used to drive the pixel in the previous time period. Dot inversion methods are characterized by a relatively superior spatial averaging because the polarity of each pixel is switched at an opposite cycle from that of its neighbors. Power consumption is greater using dot inversion than using frame or column inversion methods. In dot inversion methods, like in line inversion methods, the column driver switches the polarity of the voltage applied to the column line at the time that the information displayed on each line is updated. Further, in dot inversion methods, like in column inversion methods, the column driver switches the polarity of the voltage applied to the column at each successive cycle, and maintains every other column at an opposite polarity.
A variety of methods can also be used to apply voltage to the LCD in various embodiments such as common voltage modulation and direct drive. In the common voltage modulation, or “VCOM modulation,” the common voltage supply to the LCD is changed in order to switch the polarity of the driving voltage. For example, in the positive polarity region of the cycle, the VCOM voltage is set to 0 volt. The LCD voltage (“VLCD”), i.e., the voltage applied by the column driver to each of the columns, ranges from 0 volt to 5 volts, for example, applying a voltage of positive polarity of up to 5 volts to the LCD. In the negative polarity region, the VCOM voltage is set to 5 volts. The VLCD voltage ranges from 0 to 5 volts, as in the previous cycle. The difference between these voltages applies a negative polarity voltage of between 0 and −5 volts to the display. One advantage of the VCOM modulation method is that the driver circuitry only needs to drive the display up to half of the VLCD range in order to obtain a full VLCD range of voltage levels at the display. Thus, in the example where the VLCD ranged from 0 volt to 5 volts, and VCOM alternated between 0 and 5 volts, the total voltage that may be applied to the display is 10 volt, but the drive circuitry need only provide a range of 0 to 5 volts. A disadvantage to the VCOM modulation method is that all of the columns of the display must be driven at the same polarity. Thus, this technique is appropriate only with frame and line inversion drive methods.
In the direct drive method, the common voltage supply to the LCD is held constant. VLCD is varied from the VCOM voltage to the supply voltage. For example, the VCOM voltage may be set to 5 volts. During the positive polarity cycle, the VLCD ranges from 5 volts to 10 volt, providing a range of 0 to 5 volts for the positive half of the cycle. During the negative polarity cycle, the VLCD ranges from 5 volts to 0 volt, providing a range of from 0 to −5 volts for the negative half of the cycle. The total range is 10 volt, from −5 volts to 5 volts. An advantage of direct drive approaches is that the switching is simplified, as the VCOM does not need to be switched. Further, such approaches are readily adaptable to frame, line, column and dot inversion methods described above. One disadvantage is that the column driver circuits need to provide the full range of operating voltages.
One metric for determining characteristics of an arrangement for an LCD is the correlation of error between the difference of the output voltage of the driver and the VCOM of the driver in the positive polarity portion of the cycle, and the error between the difference of the output voltage of the driver and the VCOM in the negative polarity portion of the cycle. It is desirable for these two errors to be correlated. In other words, it is desired that the error in the negative portion of the cycle be of the same magnitude and opposite polarity as the error in the positive portion of the cycle. In this case, the errors are 100% correlated. Less that 100% correlation can induce visually noticeable results in the image.
Column driver circuitry components act as intermediaries between the digital format of the electronics that process information and the analog format of the display that presents the results to the user. Accordingly, the column driver circuitry includes a digital to analog converter component that converts the digital signals of the processing unit, bus, and memory into an analog signal. However, this analog signal must be capable of driving the liquid crystal display. While some arrangements drive the liquid crystal display columns directly from the digital to analog converter, another technique is to use a buffer interposed between the converter and the display in order to provide improved driving characteristics for the display.
While certain advantages to conventional approaches are perceived, opportunities for further improvement exist. For example, in many conventional approaches, switching the amplified signal may require relatively large switching circuitry. Larger circuitry uses substantially more area on the chip, causing increases in cost.